The mouse double minute (MDM2) oncogene enhances cancer metastasis via pathways other than the tumor suppressor p53. However, the mechanisms underlying p53-independent MDM2-mediated cancer progression remain unclear. Through the efforts to understand the p53-independent function of MDM2, MDM2 Binding Protein (MTBP) was identified. We previously demonstrated that MTBP suppresses cancer migration and metastasis independently of p53 through the following findings: 1) MTBP haploinsufficiency in mice increases metastasis of hepatocellular carcinoma (HCC), sarcoma, and other types of cancer; 2) MTBP overexpression inhibits the migration and metastasis of osteosarcoma cells lacking wild-type p53 activity; 3) MTBP endogenously binds to a migration-inducing actin-crosslinking protein alpha-actinin-4 (ACTN4) and inhibits the migration and filopodia formation mediated by ACTN4. Clinically, reduced MTBP expression in head and neck squamous cell carcinoma tissues is shown to be associated with reduced survival of patients. The goal of this proposal is to determine the roles of MTBP and its functional association with MDM2, if any, in cancer metastasis. HCC is a rising cause of cancer-related death in the United States with 5-year survival rate below 12%. The leading cause of this poor prognosis is metastatic spread. For these reasons, we will use HCC as a model for examining the contributions of MDM2 and MTBP in cancer migration and metastasis. Our preliminary results demonstrated the following: 1) reduced MTBP expression in human HCC tissues was associated with the presence of vascular/capsular invasion and lymph node metastasis, 2) MTBP inhibited HCC cell migration, 3) MTBP inhibited the filopodia formation and migration mediated by ACTN4, and 4) MTBP inhibited the activity of Elk-1, an Ets oncogene family transcription factor and a target of Erk1/2 MAP kinase. Based on these observations, we hypothesize that MTBP inhibits HCC metastasis by suppressing ACTN4 and Elk-1 activities; whereas, MDM2 promotes cancer metastasis by its inhibitory binding to MTBP. We will test this hypothesis by achieving the following two Specific Aims: Aim 1 is to determine the inhibitory effects of MDM2 on the metastasis suppression by MTBP in HCC by exploring downstream mediators of MDM2-MTBP interactions that affect metastasis; Aim 2 is to determine the in vivo contributions of MDM2 and MTBP to HCC progression in mouse models by using our recently generated hypomorphic MTBPH mice that express MTBP at only 20% of wild-type levels. Completion of this project will fill the knowledge gaps pertaining to the mechanisms by which MTBP inhibits cancer metastasis and MDM2 promotes tumor progression in a p53- independent manner through inhibition of MTBP. Given that reduced MTBP expression is found in about 70% of HCC tissues, MDM2 overexpression is detected in 30% of HCC, and metastasis is the major cause of cancer mortality, our study has the potential to significantly impact the diagnosis, treatment, and prognosis for HCC and likely other types of cancer having altered expression of MDM2 and/or MTBP.